We are engaged in a systematic analysis of cellular events in the immune response, the ontogeny of immunocompetent cells and the regulatory mechanisms that control immune responsiveness and unresponsiveness. We will use in vitro culture techniques, cell separation procedures, and a variety of reagents to characterize cell surface markers on T cells and B cells. We will examine T cell heterogeneity, i.e., how far different T cell functions reflect the activities of different subpopulations of thymus-derived cells.